Measurements to discover the presence and the concentration of ions in liquids may be accomplished electrically. A sensor is employed in which there is a conduction of electricity in proportion to the kind and concentration of ions. The amount of that conduction is measured in a conventional instrument, usually a very high input impedance voltmeter. The electrical circuit extends through the junction of several dissimilar materials as it proceeds from the liquid through the sensor to the metallic conductors of the measuring instrument and back to the liquid. A junction potential appears at each juncture of dissimilar materials the magnitude of which may approach or exceed the potentials that are generated at the sensor in its measurement of ion concentration. In that circumstance, the junction potentials must be known or the circuit rearranged to include equal but opposite junction potentials.
One of the most satisfactory means that has been devised to deal with undesired junction potentials is to employ a pair of half-cells in series with the sensing element and with one another in making the physical connection from the sampled liquid to the metallic conductors of the measuring instrument. The metal conductor of the instrument, ordinarily copper, is connected to a metal, such as silver, whose salt, such, for example, as silver chloride, readily exchanges ions with a dissolved salt with which it shares a common ion, such, for example, as potassium chloride. The potassium chloride is placed in physical contact with the sample material in which ion concentration is to be measured. The ion sensor also is placed in contact with the sample material. The electrical measuring circuit is completed from the sensor back to the instrument by placing the sensor in contact with a quantity of the same dissolved salt which contacts a quantity of the same metal salt which contact a conductor made of the metal of that same metal salt which, in turn, is connected to the metal conductors of the instrument. That arrangement includes several pairs of oppositely polarized dissimilar junctions, and it provides a means for accounting for junction potentials. However, that circuit imposes the requirement for a structural arrangement in which two quantities of salt solution can be retained while contacting the metal salt on one hand and the sample liquid and the sensor on the other hand. The physical problem of contacting the salt solution and the sensor is not very great in most cases because most sensors are formed of solid materials which can form a wall of a salt solution container. An example is the glass electrode employed in measuring pH where the pH sensitive glass forms a wall of a glass container in which the salt solution is disposed.
Finding a suitable structure which will retain a quantity of salt solution while permitting physical contact with the sample solution, without contamination or loss of one to the other, is a more difficult problem. Such structures do exist. They are called "salt bridges" in the art. There are, in fact, a wide range and variety of salt bridge structures. The great number of such structures, and the fact that the search for more structures continues, is evidence of the fact that finding a suitable structure has been difficult.
Making a salt bridge in the circumstance in which the salt solution can be permitted to flow in small quantities across the bridge requires little more than a wick. However, when flow must be restricted to near zero, forces that are otherwise neglected, or are considered on a macro-basis, become important on a micro salt. The need is to maintain an actual, physical liquid-to-liquid contact despite wide ranges in environmental conditions. Surface tension, capillary action, length of the conductive path, shape of the electric field resulting from ion concentration, and other variables that change from instant to instant as a consequence of ion migration, make it difficult to predict and design suitable bridge structures.